The operating environment within a gas turbine engine is both thermally and chemically hostile. Significant advances in high temperature alloys have been achieved through the formulation of iron, nickel and cobalt-base superalloys, though components formed from such alloys often cannot withstand long service exposures if located in certain sections of a gas turbine engine, such as the turbine, combustor and augmentor. A common solution is to protect the surfaces of such components with an environmental coating, i.e., a coating that is resistant to oxidation and hot corrosion. Coatings that have found wide use for this purpose include diffusion aluminide coatings and overlay coatings such as MCrAlY. During high temperature exposure in air, these coatings form a protective aluminum oxide (alumina) scale that inhibits oxidation of the coating and the underlying substrate. Diffusion aluminide coatings are particularly useful for providing environmental protection to components equipped with internal cooling passages, such as high pressure turbine blades, because aluminides are able to provide environmental protection without significantly reducing the cross-sections of the cooling passages.
The surfaces of gas turbine engine components are typically cleaned and, if necessary, refurbished during engine overhaul and repair. High pressure turbine components such as blades and vanes are particularly susceptible to a build up of dirt and foreign matter that must be removed to promote the service life of the component. In particular, dirt buildup within the cooling passages of a blade reduces cooling efficiency, causing increased operating temperatures for the blade. Vibratory tumbling techniques employed to clean gas turbine engine components have been successful at removing dirt from the external surfaces, but with little affect on dirt and contaminants such as silica and calcium compounds adhered to internal surfaces of components. Failure to remove silica and other contaminants from internal surfaces of components has been shown to promote hot corrosion attack during subsequent exposures to elevated temperatures.
Removal of internal dirt and contamination has generally necessitated the use of solutions, including caustics such as potassium hydroxide employed in an autoclave cleaning operation. Autoclaving has been found to be effective at removing adherent surface contaminants, but is generally not practical for field servicing due to the complexity and cost of the equipment required. Treatments with hydrofluoric acid have been successfully used to remove silica, but resulted in the formation of calcium fluoride on the treated surfaces. Other types of solutions that have found use are those containing chelating agents. A notable example is VERSENE 220, one of a series of solutions based on ethylenediaminetetraacetic acid (EDTA) commercially available under the name VERSENE from the Dow Chemical Company. Soaking in VERSENE 220 advantageously is able to loosen both silica and calcium compounds from the internal and external surfaces of a blade when ultrasonic cleaning is included. However, neutralization of VERSENE 220 with hydrochloric acid has been found necessary to prevent hot corrosion attack of the internal cooling surfaces of blades by residual VERSENE 220 during subsequent exposures to elevated temperatures. Unfortunately, the HCl treatment has been found to be aggressive toward platinum aluminide (PtAl) environmental coatings. The requirement for a hydrochloric acid treatment and resulting attack of PtAl coatings have been eliminated by the use of DIAMMONIUM VERSENE, a VERSENE solvent containing diammonium EDTA. However, proper treatment and disposal of this solution can be complicated, as is the case with VERSENE 220.
From the above, it can be appreciated that the process for removing surface buildup of dirt and contaminants on gas turbine engine components is complicated by the cleaning effectiveness, aggressiveness toward coating materials, and disposal considerations of the various solutions currently used.